246 PROCEEDINGS OP THE ACADEMY OF 



points, and a stout middle cusp bearing a stout cutting point which 

 reaches to the lower edge of the base of attachment. The lateral 

 teeth are longer and broader than the central tooth ; they are un- 

 symmetrical by the suppression of the inner cusp and cutting point, 

 and the lower half of the base of attachment being thrown off 

 towards the outer edge of the membrane, though its inner lower 

 lateral expansion is not suppressed ; the outer side cusp is well 

 developed and bears a short distinct cutting point; the inner 

 cusp is very stout and bears a very stout cutting point extending 

 slightly beyond the lower edge of the base of attachment. The 

 change from the lateral to the marginal teeth is formed by the 

 lesser size of the reflected portion and the greater development of 

 the inner cutting point, as well as by the blunt bifurcation of the 

 outer cutting point. The marginal teeth have their base of attach- 

 ment long and narrow, quadrangular, curving outward, prolonged 

 above the reflection, which is small, but bears a highly developed 

 cutting point obliquely and bluntly bicuspid on its outer edge, 

 the inner division much the larger. The number of perfect 

 laterals varies somewhat. I counted seven in citrina, eleven in 

 planilabi-um, ten in abbreviata and amanda, eight in umbilicata, 

 virginea, and bilineata, five only in gracilis. The number of 

 marginal teeth also varies, but they are numerous in all the species ; 

 in virginea I counted over one hundred and twenty. Excepting 

 that some of the membranes had narrower teeth than others, I 

 found no difference in them. They all agree (including those of 

 the species named on p. 244) with the figure given by Heynemann 

 (Mai. Blatt. 1861, t.i. fig. 1-la.) of the dentition of P. lirata. 



The genital system of one of the undetermined specimens is 

 given on pi. XIX., fig. 1. The ovary (ov.) is small and stout ; the 

 epididymis (ep.) is short and greatly convoluted at the end nearer 

 the oviduct ; the testicle (.), composed of short coeca, is small 

 and embedded in the upper lobe of the liver in the very apex of 

 the shell ; the oviduct (ovid.) is long, convoluted ; the vas deferens 

 (v. d.) enters the prostate high up on the oviduct, not at its lower 

 end, as usual ; it runs down to the external orifice, then up to near 

 the end of the penis sac, where it enters ; the vagina (v.) is long, 

 greatly swollen at the entrance of the duct of the genital bladder ; 

 the last mentioned organ (g. b.) is small, with a short duct which 

 enlarges greatly before entering the vagina ; the penis sac (p. s.) 

 is large, long, bluntly terminating, with a decided constriction 



